The invention relates to the field of repair of a cast article, and more particularly to repair of a cast article having imperfections.
Certain articles, such as airfoils for the power turbine sections of gas turbines, are constructed of superalloys. Due to the extreme temperatures and stresses to which such cast superalloy articles such as, airfoils, are exposed, imperfections in cast articles can have serious consequences. In the case of an airfoil, the control of the airfoil wall""s thickness, when an airfoil is cast, is critical to the strength and life of the airfoil.
More particularly, turbine components such as turbine airfoils are typically cast as hollow structures with complex cooling passages inside. The complex cooling passages are desired because the temperatures of the hot combustion gases directed at the airfoil during operation are at or above the melting temperature of the superalloy composition. The cooling medium may comprise air or steam. In the case of steam, such steam will typically be under pressure higher than cooling air pressure. The airfoil design is sensitive to airfoil wall thickness control because of these factors. Insufficient thickness results in significant creep damage due to high local stresses in areas of reduced thickness (i.e. cross-sectional area), while excessive wall thickness results in fatigue damage due to insufficient cooling of the exterior. An about 0.08xe2x80x3 nominal airfoil thickness is predicted to result in an airfoil achieving its design life if actual thickness can be maintained to within about 0.02xe2x80x3 of the nominal thickness.
The directional casting process for superalloy components, such as airfoils, typically utilizes ceramic xe2x80x9cbumpersxe2x80x9d on core surfaces or mold surfaces. For a nominal cast wall thickness of about 0.08xe2x80x3+/xe2x88x920.2xe2x80x3, ceramic bumpers of about 0.06xe2x80x3 in height help control wall thickness by the geometric constraints provided in the form of the ceramic bumpers. A large number of bumpers on each casting reduces the likelihood of distortion of the ceramic molds and cores.
After the article is directionally cast, the ceramic bumpers are removed. The wall of the article will be thin wherever there had been ceramic bumpers. In addition, if the mold and the core had been in contact, there would be a small hole in the article at a contact point, and a corresponding thicker area on the other side of the article. Because variations in wall thickness beyond the design specification of about 0.08xe2x80x3+/xe2x88x92 about 0.02xe2x80x3 may result in early failure of the airfoil, restoration of wall thickness is needed.
The restoration of wall thickness is complicated by the article geometry where, for example, an airfoil. At every airfoil bumper location there is a unique combination of inner surface and outer surface contours to the airfoil wall exist. In addition, due to variations in wall thickness within the design specification, no two airfoils will have the same wall thickness profile around the airfoil perimeter or along the airfoil length. An airfoil design can, for example, allow for filler material to project into the airfoil cooling cavity up to about 0.02xe2x80x3 before such filler material results in any detrimental perturbation of the coolant flow. Because the coolant can comprise steam, the inner contour at each repair should be reasonably smooth so as not to set up a site for crevice corrosion. Outer surface contours are less of a concern, since the outer surface is easily available for hand blending of the filler material back to the airfoil contour.
In addition to a need to satisfactorily repair articles due to dimensional imperfections resulting from the casting process, a further need to repair articles whereby cracks may have developed at a location on the article during the casting process exists. As well, cracks can develop in airfoils after they have been in service for a period of time. Up to now, once a crack or imperfection of sufficient size has been detected, the airfoil would be immediately replaced. If the crack or imperfection could be repaired in such a way that the airfoil could thereafter withstand the extreme operating conditions in a turbine, then the expense of replacing the cracked airfoil with a new airfoil can be avoided.
In addition to dimensional concerns for the repairing of an airfoil wall at weakened locations, the strength of the airfoil should be maintained. Steam can be a cooling medium for an airfoil, and the steam injected into the cooling passages under pressure creates a pressure vessel effect therein. A repair should be able to withstand the interior pressure and be leak-tight, and also be almost as strong as the surrounding directional material in terms of strength and resistance to creep and fatigue.
Airfoils are typically formed of superalloys that can have directionally-oriented microstructures to satisfy the mechanical strength demands for creep resistance or fatigue resistance to achieve a satisfactory design life. Directionally-oriented microstructures can be produced using directional solidification processes, which result in either elongated polycrystalline grains or single crystals. Directionally oriented microstructures may be problematic to the repair of dimensional imperfections and cracks therein, since the repaired area should have a similarly oriented microstructure of uniform strength.
Accordingly, a need exists for a method to repair a cast article having an area of imperfection in the form of a non-dimensional wall thickness. A further need exists for a method to repair a cast article having an area of imperfection, in which the repaired article in the repaired area of imperfection is capable of withstanding stresses which are substantially equal to, or a substantial amount of, the stresses which would previously be capable of being withstood in absence of such imperfection.
Further, a need exists for a method to repair a cast article, such as a hollow airfoil, having an area of imperfection or a crack, wherein the airfoil, at the repaired area of imperfection or crack, is capable of withstanding interior pressure. Still further, a need exists for a method to repair a cast article, such as a hollow airfoil, having a directionally oriented microstructure and growth axis and an area of imperfection, in which such airfoil, in the repaired area of imperfection, has an identically aligned growth axis that permits the repaired area to withstand stresses a substantial portion of those previously capable of being withstood by the airfoil.
Accordingly, in one aspect of the invention, a method of repairing a cast article having an area of imperfection, comprises the steps of forming an aperture in the cast article in the area of imperfection, the aperture having sloped side walls, forming a plug member, the plug member on an external periphery thereof provided with similarly sloped side walls, adapted to substantially abut a portion of the sloped side walls of the aperture when the plug member is disposed (inserted) therein, and inserting the plug member into the aperture. A bonding material is applied between the plug member side walls and aperture side walls, either before or after insertion of the plug member into the aperture. Lastly, the article is heated the that the bonding material bonds the sloped side walls of the plug member to the sloped side walls of the aperture.
Using a method, as embodied by the invention, an imperfection within a cast article, such as a wall of non-dimensional thickness, or a crack, may be effectively drilled out by the formation of an aperture in the place of the imperfection. By providing an aperture with sloped side walls, a plug member with similarly sloped side walls adapted to abut a portion of the sloped side walls of the aperture permits the plug member when disposed into the aperture to substantially and evenly overlie a portion of the sloped walls of the aperture. This feature aids the subsequent bonding step, since the tapered side walls, create a greater surface area for bonding than would otherwise be the case if the side walls of the aperture were non-tapered and perpendicular to the plane of the article. The feature also allows the plug to sit as close to and overlie the tapered surface to which it will be bonded without having to increase dimensional tolerances and introducing an undesirable gap, which would otherwise be the case if there were no tapered side walls and the plug member had to be machined slightly under-dimension to allow insertion into the aperture.
In another aspect of the invention, a turbine airfoil machining process produces a surface roughness, for example about 0.002xe2x80x3. Therefore, two mating machined surfaces in contact at mutual high spots can have locally variable widths in the gaps between their surfaces as wide as about 0.0004xe2x80x3. To fill a gap of about 0.0004xe2x80x3 however, a right cylindrical plug would have to be at least about 0.0004xe2x80x3 larger than the aperture, and still be capable of insertion. Pre-shrinkage of the right-cylindrical plug member by pre-cooling may not, depending on the size of the plug member, be sufficient to reduce the size of the plug member to allow interference-free insertion into a right cylindrical aperture.
In cases where pre-shrinkage is not sufficient, the force necessarily involved in pushing a right-cylindrical plug member into a right cylindrical aperture, and the difficulty in aligning the over-dimensioned plug in the aperture can cause damage to the surfaces of both the plug or the aperture. The damage can comprise undesirable scratches or micro-cracks, which could act as crack initiation sites and propagate under the inherent ambient stresses and lead to failure. The damage can also cause undesirable recrystallization upon subsequent heat. Advantageously, a method, as embodied by the invention, for forming an aperture and a plug member provides each having similarly sloped walls, the plug member may be disposed into the aperture and the sides thereof will mutually overlie and evenly abut the sloped sides of the aperture without needing to be forced into the aperture.
In another aspect of the invention, the sloped walls of the aperture and plug member allow an interference fit to be created and maintained between the sloped walls of the plug member and the aperture when the plug member is disposed in the aperture. This interference fit is created without pre-cooling of the plug member and without the difficulties inserting an oversized plug in an undersized aperture. The created interference eliminates gaps which may exist due to surface irregularities in their respective surfaces as wide as about 0.0004xe2x80x3. The interference fit also increases the bonding of the plug member to the aperture, and increases the structural integrity of the bond.
In a yet further aspect of the method of the invention, the method comprises inserting the plug member into the aperture, applying a force to the plug member to cause an interference fit between the sloped walls of the tapered plug and sloped walls of the aperture. This method allows the elimination of gaps due to surface roughness, and avoids difficulties that would be encountered in attempting to insert an over-dimensioned right cylindrical plug to a right-cylindrical aperture of lesser dimensions for an interference fit. The taper of each of the plug and aperture is sufficiently small so when forcefully inserted in the aperture, the interference fit will allow the plug member to remain therein.
Where a cast article to be repaired comprises a directionally-oriented microstructure and growth axis, the plug member should be capable of being received in the aperture. Thus, the plug member growth axis is oriented and aligned with the cast article growth axis. Thus, the plug member and aperture each possess mating engagement means, which matingly engage and allow insertion of the plug member within the aperture upon alignment of the plug member growth axis with the article growth axis. A step of inserting the plug member into the aperture comprises aligning the mating engagement means on the plug member with the corresponding mating engagement means of the aperture, and inserting the plug member in the aperture.
In another aspect of the invention, a plug member for repair of imperfections in a cast article comprises a superalloy composition and directionally-oriented microstructure. The plug member comprises a substantially identical superalloy composition and a directionally-oriented microstructure and growth axis. The plug member is provided with tapered side wall surfaces extending about its periphery and tapered to allow the plug member, when forcibly inserted within an aperture, to remain in an interference fit within the aperture. The plug member further possesses mating engagement means to matingly engage and allow insertion of the plug within the aperture when the growth axis of such plug member is aligned with the growth axis of the cast article being repaired.
The plug member of the present invention may further comprise bonding material, such as a braze alloy or other bonding material, applied to its tapered surfaces. The bonding material readies it for bonding when placed on an aperture and subsequently heated.